1. Bootable Programs Building an O/S

2. Basic Requirements of any O/S Respond to interrupts (all kinds) Preserve user environment Protect users and self from corruption Handle I/O errors Handle user errors

3. Responding to ANY interrupt Hardware loads a pre-defined state vector – Context switch occurs – NSI is inside kernel at primary interrupt handler Primary handler determines type of interrupt – I/O completion – Service call – Program error – Paging/addressing exception Save user’s environment (stack, registers) in process’s PCB or thread’s TCB Branch to detail interrupt handler

4. Detail interrupt handler Take required action – All instructions valid – Must not cause another interrupt Therefore must not do I/O - one exception: – Handling I/O errors may require I/O to device – Interrupt for this 2ndary I/O must be deleted Must allow for all possible (and improbable) errors Return to common return point

5. Common return to user Determine next thread/process to run – New user – Interrupted user Locate PCB/TCB for selected user Prep state vector, could be: – Old vector (in reserved RAM) from previously interrupted program – New vector data determined for a new program Save kernel’s registers in reserved storage Restore user’s registers & stack from PCB/TCB Load prepared state vector for user Context switch occurs

6. SOS A Small Operating System “built-in” user command-line shell Written in z/390 assembler language – 32-bit mode – Runs in a z/390 virtual machine You will – Modify it to provide service call handling – Install it as a bootable program – Boot it – Test it

7. SOS – bootable drive Virtual device at address 222 Preformatted as a CMS disk – Simplifies putting programs on the drive – Reserved space for the bootable program (SOS) – Boot loader (IBM supplied) installed by student – Allows seeing disk content from CMS shell

8. SOS – original contents Device address discovery – Operator’s terminal – Boot drive Locate volume TOC Locates file directory Displays content of directory in hexadecimal Terminates

9. SOS - Lab 1 Copy the SOSSAMP file to your “a-disk” Rename it to: SOS Assemble it using HLASM Prepare the bootable drive Boot it

10. SOS – Lab 2 Add “shell” code to the single compile Add state-vector loading to “activate” shell Shell displays your prompt, waits for input Shell issues SVC n in response to input – 0<= n <= 7 – See lab assignment for details SOS gets control and performs service “n” SOS – returns control to shell – loads “terminate” state vector